Pharmaceutically active drugs can exist in different solid forms. For example, a drug may exist in different crystalline forms which have different physical and chemical properties.
Different physical properties can cause different crystalline forms of the same drug to have largely different processing and storage performance. Such physical properties include, for example, thermodynamic stability, crystal morphology [form, shape, structure, particle size, particle size distribution, degree of crystallinity, color], ripple behavior, flowability, density, bulk density, powder density, apparent density, vibrated density, depletability, emptyability, hardness, deformability, grindability, compressability, compactability, brittleness, elasticity, caloric properties [particularly melting point], solubility [particularly equilibrium solubility, pH dependence of solubility], dissolution [particularly dissolution rate, intrinsic dissolution rate], reconstitutability, hygroscopicity, tackiness, adhesiveness, tendency to electrostatic charging, and the like.
In addition, different chemical properties can cause different crystalline forms of the same drug to have largely different performance properties. For example, a crystalline form having a low hygroscopicity (relative to other crystalline forms) can have superior chemical stability and longer shelf-life stability (cf. R. Hilfiker, Polymorphism, 2006 Wiley VCH, pp 235-242).
Further, different stereoisomers of one compound can form different crystalline forms. In some cases this difference can be exploited to allow separation of the stereoisomers from one another.
One particular drug that is of great interest for use in treating cancer pain (and other acute, visceral, neuropathic and chronic pain disorders) is (1r,4r)-6′-fluoro-N,N-dimethyl-4-phenyl-4′,9′-dihydro-3′H-spiro[cyclohexane-1,1′-pyrano[3,4b]indol]-4-amine. This drug is depicted below as the compound of formula (I).
